Many of today's mobile terminal users have subscribed to more than one mobile communication service. For instance, such a user could have one private and one work-related subscription. In another case, such a user could have two different subscriptions for use in different areas (e.g., countries).
Accordingly, dual SIM terminal devices become increasingly popular. A dual SIM device is typically realized in the form of a mobile terminal which is capable of holding two SIM cards. Dual SIM operation allows the use of two subscriptions without the need of carrying two mobile terminals. For example, the same mobile terminal can be used for business and private purposes with separate numbers (and bills) or for travel, with an additional SIM for the country visited.
The most common dual SIM terminal devices are those of the dual standby-type, i.e., the device can—in standby—camp on two public land mobile networks (PLMNs) or cells, one for each active SIM. Furthermore, it is expected that multi-standby SIM devices capable of handling more than two SIM cards will be on the market soon.
However, in such a dual standby SIM device, only one subscription (and associated SIM) can be active at a time. That is, once one of the subscriptions is activated, the other subscription is deactivated. Hence, as long as one of the subscriptions is in an active state, the device can not be reached (or transmit) via the other subscription. This is attributed to the fact that a dual standby device only has one transceiver chain, and in case of active use of one subscription (and associated SIM), the entire transceiver chain is assigned to that subscription.
This fact especially poses a problem for data or e-mail services, for which the device can be in an active state quite a long time after actual transmission and reception operations have taken place. For instance, in Wideband Code Division Multiple Access (WCDMA), the first subscription can be in the Cell Forward Access Channel (CELL_FACH) state in the order of minutes (depending, e.g., on network parameters) before transiting to an idle state. During this time, the second subscription may not be used. However, from a user's point of view, substantially no differences between the CELL_FACH state and an “idle state” are perceivable.
One naïve approach to solve this problem may reside in providing two transceiver chains in the dual SIM device and in assigning one chain to each subscription. However, this is not a cost efficient solution.